We have now promoted the Improved Bookmarks with Reminders to a core feature, available to everybody, with the new 2.5.0.beta3. All existing post_actions bookmarks will be migrated when you update your Discourse forum. Feedback is encouraged and welcome! 
I have updated the OP with a more accurate description of the features. The following important changes have been made recently:
- Bookmarks with reminders can now be edited.
- Deleting bookmarks is now a 2-step process completed from the modal window, and a confirmation is shown if you are deleting a bookmark with a reminder.
- Keyboard shortcuts have been added to select a reminder type and navigate the bookmark modal.
- “Later Today” is now available until 5pm, and the later time is capped to 6pm. So from 3pm-5pm you will be able to see and use this reminder option but it will not go later than 6pm.